Editors’ Highlights

A New Spin on Grain Segregation in Fault Zones

Fine-grained layers in sheared fault gouge may be formed by shear-driven size-segregation in granular materials, rather than by shear localization.

Source:
Geophysical Research Letters

Shear experiments in a rheometer. Panel a shows a sheared quartz sand with crushed particles (red arrows) accumulating away from the shear boundary (the bottom of the steel rotor, denoted by a red dashed line). Panel b shows the sheared material resulting from an upside-down experiment, in which the maximum shear rate is at the bottom (red dashed line), and the crushed, smallest grains are at the center of the volume (red arrows). Panel c shows the actual shear rate profile for this experiment (red line), plotted next to a hypothetical profile that might naively be inferred based on the particle size distribution in b. Credit: Siman-Tov and Brodsky, 2018, Figure 2

Geologists commonly observe layers in fault zones that are thought to be a biproduct of the shearing process: finer-grained material is created as the rocks in the shear zone grind against each other. Geophysicists and geologists often interpret the formation of fine-grained layers in sheared fault gouge as evidence of slip localization within a narrow plane where fine-grained materials are produced and accumulate. Siman-Tov and Brodsky [2018] revisit this concept. Using experiments in an annular shear cell (rheometer) setup, they show that the thin fine-grained layer is formed by shear-driven size-segregation in granular materials, and not by localization in the thin layer; fine materials segregate farther from the boundary, where granular shear rate is low. This work thus has the potential to change the way we interpret shear/slip localizations in fault zones.

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